Effect of Ca-doping on the electronic and magnetic properties of orthorhombic and hexagonal YMnO₃ thin films

The intriguing and distinctive orbital and magnetic ordering exhibited in orthorhombic and hexagonal multiferroic YMnO₃ have been regarded as issues of fundamental importance in understanding the underlying magnetoelectric effect. In particular, recently, coexistence of E-type and cycloidal antiferromagnetic (AFM) ordering types was observed in some orientation-engineered orthorhombic YMnO₃ (o-YMO) films and has been attributed to the local strain introduced in the MnO₆ octahedral. On the other hand, it has been generally conceived that the MnO₅ triangle lattice arrangement in hexagonal YMnO₃ (h-YMO) may have resulted in frustrated magnetic structure due to the competitive interaction between spin and complex magnetic order. In this study, aiming at achieving further control and modification on this technologically important material, we deliberately grew the o-YMO and o-Y_0.93Ca_0.07MnO₃ as well as h-YMO and h-Y_0.93Ca_0.07MnO₃ thin films, respectively, on SrTiO₃(110) and YSZ(111) single crystal substrates by pulsed laser deposition (PLD). The crystal structure and relevant lattice strain of the thin films were characterized by X-ray scattering, including θ-2θ, Φ-scans and reciprocal space mappings. The normalized XANES spectra at the Mn K-edge were conducted to characterize the effects of Ca-doping on the local electronic structures at the Mn site of the samples. The temperature dependent susceptibility χ(T) revealed evidences showing that the spin reorientation of the AFM order varied at low temperatures in both o-YMO and h-YMO thin films. Nevertheless, the details of the the magnetic characteristics were very different and complex, which might be also relevant to the specific crystalline structure in discussion. Moreover, the M-H hysteresis loops taken at various temperatures exhibit significant changes with Ca-doping in both o-YMO and h-YMO films, suggesting that the extra carriers resulted from Ca-doping might have substantially modified the exchange coupling between the magnetic moments of Mn ions and even domain wall properties of the AFM orders.